JAMB Chemistry 1990

C: Dissolving sodium chloride in water

When sodium chloride dissolves, its ions disperse through the water. Evaporating the water can recover the salt, so this is classified as a physical change. The other processes involve chemical reactions that produce new substances.

A: Sublimation, condensation and freezing

X goes directly from solid to gas, which is sublimation. Y goes from gas to liquid, which is condensation. Z goes from liquid to solid, which is freezing.

C: 32 g of O₂

Using the relative atomic masses Mg = 24 and O = 16, 24 g of Mg contains one mole of atoms. A mole of oxygen molecules has mass 32 g, so it contains the same number of particles. The other choices each contain half a mole of molecules.

D: CO, H₂S, NO₂, SO₂

The relative molecular masses are CO = 28, H₂S = 34, NO₂ = 46 and SO₂ = 64. In the stated approximation, rate is proportional to 1/√M, so lighter gases diffuse faster. The order is CO, H₂S, NO₂, SO₂.

D: S

S has 19 protons and 20 neutrons, so its mass number is 39. Atomic number 19 is odd and identifies potassium, which commonly loses one electron to form K⁺. The other atoms do not satisfy all three conditions.

B: Valence

Valence, or valency, describes an atom’s combining capacity, commonly expressed as the number of bonds it forms in simple compounds. Atomic number counts protons, oxidation number is a formal electron-accounting value, and electronegativity describes attraction for bonding electrons.

C: 4.70%

Pb(NO₃)₂ + H₂S → PbS + 2HNO₃ gives one mole of PbS per mole of H₂S. The PbS molar mass is 207 + 32 = 239 g mol⁻¹, so the H₂S amount is 5.02/239 mol. Its STP volume is about 0.470 dm³, giving (0.470/10.0)×100 = 4.70% by volume.

C: Mercury(II) chloride

Mercury used as a cathode can enter process wastes. Some is oxidised to mercury chloride compounds, including mercury(II) chloride. This makes mercury compounds a characteristic contamination risk of the mercury-cell chlor-alkali process.

B: Solid or liquid particles dispersed in gas

Smoke is an aerosol: small solid particles and often liquid droplets are suspended in a gas. Soot particles and condensed droplets are dispersed through air, which acts as the continuous phase.

C: 100 g mol⁻¹

The 25.00 cm³ aliquot contains 0.02000×0.1 = 0.00200 mol of acid because the acid is monobasic. The full 250 cm³ contains ten times as much, or 0.0200 mol. Its molar mass is 2.0/0.0200 = 100 g mol⁻¹.

A: 4.0×10⁻⁵ mol dm⁻³

For the usual dilute-solution calculation, [H⁺] = 10⁻ᵖᴴ. With pH 4.398, [H⁺] = 10⁻⁴·³⁹⁸ ≈ 4.0×10⁻⁵ mol dm⁻³. A change of one pH unit corresponds to a tenfold concentration change.

A: 0.20 mol

Deposited nickel is proportional to charge. Increasing from 0.1 to 0.4 faraday deposits four times the mass: 4×2.95 = 11.8 g. The amount is 11.8/58.7 ≈ 0.20 mol.

B: +6 to +3

For Cr₂O₇²⁻, let chromium’s oxidation number be x: 2x + 7(−2) = −2, so x = +6. The product is Cr³⁺, whose oxidation number is +3. Chromium is therefore reduced from +6 to +3.

C: −848 kJ

Use formation enthalpies of products minus those of reactants. Aluminium and iron in their elemental reference states contribute zero. Thus ΔH = [−1670 + 2(0)] − [−822 + 2(0)] = −848 kJ. There is one mole of Al₂O₃ in the balanced equation.

A: Zinc has a more positive oxidation potential than iron

Zinc is oxidised more readily than iron and acts as the sacrificial anode. Its oxidation potential is more positive than iron’s, while its reduction potential is more negative. Electrons supplied by zinc oxidation keep the iron cathodic and help protect it from corrosion.

D: 5 g of powder at 50 °C

Powder exposes more calcium carbonate surface to the acid than lumps do. A higher temperature also increases the rate by increasing the frequency of effective collisions. Powder at 50 °C combines both rate-increasing factors.

D: Cl₂

Chlorine is denser than air and can enter below it, pushing air upward out of an upright gas jar. This is upward displacement of air. Hydrogen and ammonia are lighter than air; nitrogen monoxide reacts with oxygen in air and is unsuitable for this collection method.

C: Expose each gas to air

Nitrogen monoxide, NO, reacts with oxygen in air to form brown NO₂: 2NO + O₂ → 2NO₂. This visible change distinguishes NO from the other three colourless gases in the list. Limewater alone cannot distinguish CO₂ from SO₂ because both can form white precipitates.

C: Iron

The conventional Haber process uses an iron catalyst to speed the reaction between nitrogen and hydrogen. A finely divided catalyst provides a large surface on which the reactant gases can adsorb and react.

C: Carbon monoxide can accumulate and cause poisoning

Incomplete combustion of coal can produce carbon monoxide. CO binds strongly to haemoglobin and reduces the blood’s ability to carry and release oxygen; it also interferes with cellular respiration. Dangerous CO can accumulate in a poorly ventilated room.

D: Calcium silicate

In the usual blast-furnace explanation, limestone supplies CaO, which reacts with silica impurity: CaO + SiO₂ → CaSiO₃. The calcium silicate joins the slag and is separated from the molten iron.

A: It readily absorbs water vapour from air

Solid sodium hydroxide absorbs moisture from air. It is strongly hygroscopic and can become wet as enough water is absorbed, changing the condition and concentration of the stored reagent. A tightly closed container limits this uptake.

D: Copper

Copper compounds can introduce colour into glass. Copper ions interact with light, producing characteristic colours that depend on composition and oxidation state. Potassium, barium and zinc do not provide the same characteristic transition-metal colouring in this comparison.

C: 2,4-Dimethylhex-3-ene

The longest chain containing the double bond has six carbons. The double bond is at position 3 from either end. Numbering from the nearer branched end gives methyl groups at positions 2 and 4, rather than 3 and 5, so the name is 2,4-dimethylhex-3-ene.

C: Alkanol

An alkanol has an O–H group that reacts with sodium to form an alkoxide and hydrogen gas. Alkanols are ordinarily neutral to litmus. Among the four listed classes, this combination of observations identifies an alkanol.

B: Neutralisation

Sodium hydroxide removes one acidic proton from oxalic acid. The products are sodium hydrogen oxalate and water: HOOC–COOH + NaOH → HOOC–COONa + H₂O. This acid–base reaction is neutralisation.